The present invention relates generally to the field of gas concentration analysis. More specifically, it relates to an apparatus and a method for measuring the concentration of selected gases (i.e., oxygen and carbon dioxide) in the exhaled breath of an animal, particularly a human patient who is breathing with the assistance of a ventilator.
Clinical treatment of a ventilated patient often calls for an analysis of the oxygen and carbon dioxide concentration in the patient's arterial blood. In recent years, invasive techniques have given way to proceduresinvolving an analysis of gas concentrations in the patient's exhaled breath, there being a known correlation between the carbon dioxide and oxygen concentrations in the exhaled breath and in the arterial blood.
The most accurate determination of blood gas concentration is obtained when the carbon dioxide level in the exhaled breath reaches a plateau level. The portion of the exhaled breath where this plateau is reached is known as the "end-tidal region". The focus of the prior art has, therefore, largely been toward gas analysis systems that sample the patient's exhaled breath within the end-tidal region.
For example, U.S. Pat. No. 3,910,261 to Ragsdale et al. discloses an expiratory gas analyzer that is electronically-controlled, by means of inspiratory and expiratory flow sensors, to sample only the end-tidal portion of the patient's exhaled breath. In U.S. Pat. No. 4,423,739 to Passaro et al., an expiratory gas analyzer is disclosed that produces an output signal indicative of carbon dioxide concentration throughout exhalation, but processes only that portion of the signal representing the end tidal region.
While prior art gas analyzer systems, such as those described above, have proven very useful for adult patients, they have been less effectively used with small children, particularly infants. This is because infants typically breathe much more rapidly than adults, and each breath is much smaller. Typical gas analyzer cells cannot make accurate measurements with such rapid, low volume breaths. To solve this problem, several prior art systems have resorted to accumulating several successive exhaled breath samples from the patient and then releasing the accumulated multiple breath samples to a gas analysis cell. See, for example, U.S. Pat. No. 4,202,352 to Osborn; U.S. Pat. No. 4,346,584 to Boehringer; and U.S. Pat. No. 4,619,269 to Cutler et al.
Nevertheless, the prior art has still not completely addressed all of the requirements for accurate gas concentration analysis, particularly in pediatric applications. Specifically, the pediatric devices of the prior art, as typified by the above mentioned Osborn, Cutler et al, and Boehringer patents, lack sufficiently precise control of the sampling period to assure that only end-tidal samples are taken. Moreover, sampling period control techniques that depend on sensing the starting and cessation of relatively large inspiratory and expiratory flows, as exemplified by the Ragsdale et al. patent, supra, while appropriate for volume-cycled, demand-responsive adult ventilators, are not suited for continuous flow, time cycled pediatric ventilators.
There has therefore been a long-felt, but as yet unsatisfied need for an exhaled gas analysis system that is particularly adapted to pediatric applications. More specifically, there is a need for an exhaled gas analyzer system that provides for precise control of the sampling period to assure the extraction of samples only during the end tidal region, without relying on the sensing of relatively large respiratory gas flows. Moreover, such a system must be compatible with the time-cycled operation of typical pediatric ventilators and it must be able to achieve accurate gas concentration measurements from the rapid, low volume exhalations of infants.